Fluid pressure regulating PWM electromagnetic valves generally comprise a magnetic armature element, a spring or other resilient means for biasing the armature into engagement with a rest seat, and a magnetic circuit that is adapted when activated to develop an armature attracting force which opposes the spring force and moves the armature into engagement with an active seat. In a normally open valve, the rest seat is referred to as the exhaust seat, the active seat as the supply seat; in a normally closed valve, the rest seat is referred to as the supply seat, the active seat as the exhaust seat.
In valves of the above type, the armature and active seat must exhibit magnetic characteristics since they form part of the magnetic circuit. However, response considerations dictate against direct contact between an active magnetic seat and armature. With such direct contact, the armature and active seat tend to become residually magnetized, requiring relatively high spring force to return the armature to its rest position upon deenergization of the magnetic circuit. High spring forces are also undesirable since higher magnetic forces are required to unseat the armature at each energization of the magnetic circuit. Both residual magnetism and high spring forces are particularly undesirable in valves intended for high speed modulation.
One technique for overcoming the response tradeoff described above is to coat the contacting surfaces of the armature or seat with a nonmagnetic material such as copper. The coated element retains its magnetic characteristics, and the residual magnetism problem is avoided because the coating creates an effective magnetic air gap between the armature and seat. However, this technique tends to degrade the durability of the valve because the coating is generally relatively soft, and tends to wear out long before the useful life of the valve, especially when the contacting surfaces cannot be rotated or indexed to distribute the wear.
Another technique for overcoming the response tradeoff is to fasten a nonmagnetic insert on the active seat. However, this technique is undesirable since it significantly increases the cost and complexity of assembly.